1. Field of the Invention
The present invention relates to substrates for liquid crystal display devices and particularly to an improvement of signal lines and nonlinear elements formed on insulating substrates.
2. Description of the Background Art
So-called active type liquid crystal display devices provided with transistors or diodes corresponding to respective pixel electrodes for high multiplexing drive of liquid crystal elements arranged in a matrix are presently manufactured. MIM (metal-insulator-metal) type nonlinear elements which can be manufactured relatively easily are often used as elements for such pixel electrodes.
FIG. 1 is a schematic sectional view of a liquid crystal display device including MIM elements, and FIG. 2 is a plan view showing a part of the lower substrate of the liquid crystal display device of FIG. 1. Referring to those figures, a first conductor pattern 12 serving as signal lines 12a, pixel electrodes 15, and a second conductor pattern 14 connecting the first conductor pattern 12 and the pixel electrodes 15 are formed on the first insulating substrate 11 of glass or the like. A scanning electrode 17 is provided on the lower surface of a transparent second insulating substrate 16, and a liquid crystal layer 19 is enclosed between the first and second substrates 11 and 16 spaced by spacers 18.
An MIM element has a laminated structure including a first metal layer, an insulator layer, and a second metal layer, and as can be seen from FIGS. 1 and 2, a portion 12b of the signal line 12a is usually used as the first metal layer. A thin oxide film (not shown) formed on the surface of the first metal layer 12b is used as the insulator layer. The second conductor pattern 14 covering the insulator layer and connected to the corresponding pixel electrode 15 is used as the second metal layer. In this case, the signal lines 12a, 12b are often formed by tantalum because of easy patterning, good adherence of anodized films formed as the insulator layers, or the like.
However, native oxide films are liable to be formed on the surfaces of the signal lines 12a of tantalum, which causes unfavorable increase of electric resistance values in connections with external circuits.
If the liquid crystal elements are to be activated in order to enhance the contrast of the liquid crystal display device, it is desirable to flow sufficient ON current in the MIM elements. If the liquid crystal elements are to be inactivated, it is desirable to limit OFF current to as small a value as possible. However, if an anodized tantalum oxide (TaO.sub.x) film is to be used as the insulator layer of the MIM element, it is not possible to sufficiently reduce the OFF current in the MIM element and it is difficult to obtain a satisfactory contrast value for the liquid crystal display device.